Boron dynamics in volcanic ash-derived soils from the Pacific coastal plain of Guatemala
Terraza Pira, Maria Fernanda
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Boron (B) is an essential trace element for plants and animals, and its range between deficiency and toxicity is the narrowest among all the micronutrients in soils. Volcanic ash-derived soils naturally have low B contents, and crops growing in these regions suffer severe symptoms of B deficiency that compromise their yields and quality. B availability in soils is governed by adsorption and desorption reactions on mineral and organic functional groups, and a deep understanding of the surface chemistry of these components is essential to successfully managing this nutrient at the field scale. The purpose of this study was to understand the physicochemical reactions of B with soils from the Pacific coastal plain of Guatemala so that recommendations for economically important crops in the region could be improved. An organomineral characterization of a set of 23 soils belonging to five different soil orders revealed B is adsorption is controlled by amorphous aluminosilicates (allophane and imogolite), Fe oxy(hydroxydes), 1:1 clay minerals, and recalcitrant fractions of organic matter. The maximum B adsorption capacities for these soils —that are geographically close but differ greatly in their chemical properties— were obtained from Langmuir isotherms of the data and varied from 3 to 130 mg B/kg; with the degree of reversibility of the adsorption reactions depending on the andic character of the soil. Because the andic character of a soil can be estimated by a measurement of pH in a sodium fluoride (NaF) suspension, this test is tentatively proposed for predicting the potential of a soil to fix B. An estimation method to calculate appropriate field B rates for highly fixing soils based on adsorption equilibria is proposed. When these estimated B rates were applied in the field, deficient soil levels were improved and maintained in sufficiency ranges without causing toxicity during the elongation stage of sugarcane, which was the response crop model used to test the rates in two commercial farms in Guatemala. The increased plant absorption and improved yields demonstrate that the correct management of B has good potential to positively impact agricultural productions in Central America and other volcanic regions in the world.
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